Mining is an essential industry, producing many valuable commodities that form the basis of the world economy, but with a history of negative environmental consequences. Mine waste streams from projects with sulfide ores are particularly detrimental, as both unprocessed waste rock and processed tailings material typically contain significant amounts of unoxidized or partially oxidized sulfide minerals. These minerals, over time, will react with water and atmospheric oxygen to create sulfuric acid and dissolved metals, a problem known as Acid Rock Drainage or Acid Mine Drainage. This can contaminate waterways and groundwater, leaving a long-term environmental problem.
Acid Rock Drainage (ARD) from mine waste rock, tailings, and mine structures such as open pits and underground workings is primarily a function of the mineralogy and permeability of the rock material and, as noted above, the availability of water and oxygen. ARD occurs naturally and as consequence of various mine activities. ARD, within this context of mining activity, may be referred to as Acid Mine Drainage (AMD), a subset of ARD.
Within a mine site, as field conditions during operation and long term storage are highly variable and difficult to assess in advance, predicting the potential for ARD is currently challenging, expensive, and of questionable reliability. ARD from mining operations is a costly problem and one in which both mine operators and governments alike are seeking solutions. In addition to the acid contribution to surface waters, ARD may cause metals such as arsenic, cadmium, copper, lead, mercury, and zinc to leach from mine wastes. This metal load causes environmental damage, and may be of greater concern than the acidity in environmental terms. Despite rigorous engineering and design, impoundment and treatment of acidic metal-bearing waters and/or sulfide-bearing materials can be compromised by human error, mechanical breakdowns, unrecognized geological features or extreme weather events.
Wastes that have the potential to generate acid as a result of mining activity include mined material such as tailings, waste rock piles or dumps, and spent ore from heap leach operations. While not mined wastes, pit walls of surface mining operations, mineralized areas left in underground mines and stockpiled ore also have the potential to generate ARD.
Simply put, acid is generated when metal sulfide minerals are oxidized. Metal sulfide minerals are present in ore bodies and surrounding host rocks at many mines and un-mined mineral prospects. Oxidation of these minerals and the formation of sulfuric acid occurs through natural weathering processes, however the oxidation rates of undisturbed ore bodies and release of acid and mobilization of metals is usually slow due to low permeability and natural buffering reactions. Thus, discharge from such undisturbed deposits poses limited threat to receiving aquatic ecosystems, which have usually adapted to the naturally elevated levels of ARD components if present.
Extraction operations associated with mining activity can greatly increase the rate of these oxidation reactions by exposing large volumes of sulfide-bearing rock material, with increased surface area, to air and water. The oxidation of sulfide minerals consists of numerous reactions and each type of sulfide mineral has a different oxidation rate. For example, pyrrhotite, marcasite and framboidal pyrite will oxidize quickly while crystalline pyrite will usually oxidize more slowly. Common sulfide minerals are identified in Table 1.
TABLE 1Partial List of Sulfide Minerals1MineralCompositionPyriteFeS2MarcasiteFeS2ChalcopyriteCuFeS2ChalcociteCu2SSphaleriteZnSGalenaPbSMilleriteNiSPyrrhotiteFe1−xS (where 0 < x < 0.2)ArsenopyriteFeAsSCinnabarHgS1Ferguson, K. D. and P. M. Erickson, 1988. Pre-Mine Prediction of Acid Mine Drainage. In: Dredged Material and Mine Tailings. Edited by Dr. Willem Salomons and Professor Dr. Ulrich Forstner. Copyright by Springer-Verlag Berlin Heidelberg 1988.
The primary factors governing acid generation include the particular sulfide minerals present, moisture content, oxygen levels, permeability, ambient temperature, concentration of ferric iron, and in some cases the presence of bacteria which can catalyze the oxidation reactions. Also important is the physical occurrence/type of sulfide mineral. Large, well crystallized (euhedral) minerals have smaller exposed surface areas than a similar volume of irregularly shaped, finer grained minerals, and thus react less rapidly.
Furthermore, as ARD contains sulphuric acid, the pH of the contaminated runoff, (runoff that stems from contact between sulphide minerals and exposure to air and water) continues to decrease with ongoing sulphide oxidation. Under these low pH conditions, ferric sulphate may be oxidized to ferric iron, which is capable of oxidizing other minerals such as lead, copper, zinc or cadmium sulphides. As a result, ARD frequently contains high concentrations of toxic dissolved metals.
It is clear that both water and oxygen are necessary to generate acid drainage. Water serves as both a reactant and a mechanism for transporting oxygen and aqueous products. A ready supply of atmospheric oxygen is required to drive the oxidation reaction.
Mitigation of ARD is often performed by immersing waste products in water, or capping them with an impermeable layer, both of which are intended to prevent oxygen from reaching the reactive materials. These methods are expensive, and require on-going maintenance and oversight for decades after a project ceases operation. The risk of long-term environmental damage and cost of a decommissioning project can be greatly decreased by a process which more rapidly converts all or most of the sulfide minerals to chemically stable forms. There is a need for a better, more efficient and more economical process.
Active mine projects are also significant consumers of electricity, with beneficiation processes in particular being energy intensive, and they often require heat for buildings or processing steps. Many former mine sites are still connected to electrical grids by under-utilized transmission lines. In many regions there is not enough existing generating capacity to supply electrical power demand of mines and grids to which they are connected, and new thermal power plants are planned to satisfy this demand. Power and heating plants often burn hydrocarbons such as coal, oil, or natural gas, which produce emissions that contain significant amounts of CO2, a known greenhouse gas. CO2 sequestration, a process by which CO2 is locked away in a form which removes it from the atmosphere, is becoming increasingly important as governments world-wide become concerned about climate change.
The Faro Mine in Yukon, Canada, is one example of a site left in an environmentally unsound state when the operator went bankrupt. This project is currently being decommissioned and is expected to cost the Canadian Federal government over $700M to clean-up over a period of 25 years. The Faro clean-up includes capping all reactive waste under impermeable covers which will prevent oxygen from reaching it. If these covers are ever damaged, the material will begin to react again. A means to accelerate this process in a controlled environment would be hugely beneficial.
Accordingly there is a need across varying mining industries, for a treatment system, in particular one that is adaptable to in situ operation and wherein sulfide-rich waste is treated to reduce environmental impacts including ARD and wherein valuable reaction products are also obtained.
It is an object of the present invention to obviate or mitigate all of the above-noted disadvantages.